1. Field of the Invention
The present invention relates to image processing apparatus and method for optimizing an image formation condition in accordance with states of an image formation apparatus.
2. Related Background Art
Hereinafter, conventional art will be explained with reference to accompanying drawings.
FIG. 7 is a sectional view showing a multi-color image formation apparatus. As shown in FIG. 7, in the multi-color image formation apparatus, a photosensitive drum 1 and a charge unit 3 are provided. Further, on a left side of the drum 1, a plurality of development units 4a, 4b, 4c and 4d are supported by a rotatable support unit 4. Furthermore, on a right side of the drum 1, there is provided an intermediate transfer body 5 which supports a plurality colors of toner images simultaneously. In such structure, the photosensitive drum 1 is rotatively driven in a direction indicated by an arrow, by a not-shown drive means.
On an upper side in the multi-color image formation apparatus, there are provided a laser diode 12 which constructs an exposure unit, a polygon mirror 14 which is rotatively driven by a high-speed motor 13, a lens 15, and a reflection mirror 16.
In a case where a signal representing an yellow (to be referred as "Y" hereinafter) image is input into the laser diode 12, light information corresponding to the Y image is irradiated onto the photosensitive drum 1 via an optical path 17, and thus a latent image is formed. Further, if the photosensitive drum 1 rotates in the direction indicated by the arrow, the formed latent image is visualized by the development unit 4a using a Y toner. The toner image on the drum 1 is then transferred onto the intermediate transfer body 5.
By performing such a process also for magenta (to be referred as "M" hereinafter), cyan (to be referred as "C" hereinafter) and black (to be referred as "Bk" hereinafter) respectively, a full-color image using a plurality colors of toners is formed on the intermediate transfer body 5. Subsequently, when the toner image including a plurality of colors on the intermediate transfer body 5 reaches a transfer portion at which a transfer charge unit 6 is arranged, the toner image on the intermediate transfer body 5 is transferred onto a transfer member. In this case, the transfer member has been supplied to the transfer portion before the toner image reaches. Further, the toner image on the transfer member is melted to be fixed to the member by a fixing unit 9, thereby obtaining the color image.
On the other hand, the residual toner on the photosensitive drum 1 is cleaned away by a cleaning unit 11 such as a fur brush, a blade or the like. Also, the toner on the intermediate transfer body 5 is cleaned away by a cleaning unit 10 such as a fur brush, a web or the like. In this case, the cleaning unit 10 eliminates the toner by rubbing a surface of the intermediate transfer body 5.
In the above-described multi-color image formation apparatus, if an image density varies according to various conditions such as use circumstances, the number of prints and the like, an inherent and correct tone (i.e., color tone) can not be obtained. Therefore, conventionally, in order to judge a state of the image in case of the image formation, a toner image (to be referred as "patch" hereinafter) to be used for detecting each color density is experimentally formed on an image support body to automatically detect such the density. Subsequently, a detected result is fed back to a process condition, e.g., light exposure quantity, development bias and the like, and to an image formation condition which is controlled by a color process, e.g., gamma correction or the like. Thus, density control is performed to form the inherent color image, and to obtain the stable image.
In this case, as shown in FIG. 9, there has been popularly known as a patch formation method a method in which the patches of the respective colors are sequentially formed from an image formation start position and all the formed patches are held within a printing region.
Conventionally, as a density sensor 2 for detecting a density of the patch, there has been frequently used a type having structure shown in FIG. 8. That is, a light emission element 102 irradiates an infrared light onto the Y, M, C and Bk toners (or toner patches) on the intermediate transfer body 5, by using an infrared LED. Then, a reflected light from the intermediate transfer body 5 is detected by a light reception element 101 to be converted into an electrical signal. In the density sensor 2, the light emission element 102 is arranged at an angle different from that of the light reception element 101, such that the light reception element 101 measures an irregular reflection light from patches 105A and 105B. Such the structure has an advantage that a pair of the light emission element 102 and the light reception element 101 can detect all the Y, M, C and Bk toners, but has a disadvantage that an output characteristic of the density sensor 2 for the Y, M and C toner patches is different from that for the Bk toner patch. For this reason, in case of converting sensor outputs into densities, sequence in a process for the Bk toner must be made different from sequence in processes for the Y, M and C toners.
As structure of an another density sensor, there is supposed the structure in which the three colors of light emission elements corresponding to respective spectra of the Y, M and C toners and the corresponding three colors of light reception elements are provided to detect the densities of the patches respectively corresponding to the Y, M and C toners. Such the structure has an advantage that all of the four kinds of toners have the same output characteristic of the density sensor. For this reason, there is only one sequence in the process for converting the sensor output into the density. On the other hand, such the structure has a disadvantage that three pairs of the light emission and reception elements are necessary, whereby a manufacturing cost is significantly increased and also a size of the entire apparatus becomes large. For this reason, such the structure is hardly used in the density sensor which is arranged in the multi-color image formation apparatus.
In any case, when the residual toner on the intermediate transfer body 5 is cleaned away, such the residual toner can not be completely eliminated. Therefore, there has been confirmed by the inventors of the present invention that the residual toners are gradually accumulated and thus causes a change in a surface color of the intermediate transfer body 5, thereby degrading a reflectance. Further, there has been confirmed that, since a result of the density detection is remarkably influenced by the reflectance (i.e., reflectance for a light used for detecting the density) of a substrate (or background) on which the toner is attached, there has been occurred a problem that a density measurement value varies as time elapses.
FIG. 6 shows a relation between the toner density and the density sensor output in a case where the patch is measured at each of positions A, B and C on the intermediate transfer body. In this case, the reflectances of the positions A, B and C are different from others, and become lower in order of the positions A, B and C. Further, the same measurement result can be obtained respectively for the Y, M and C toners, whereby FIG. 6 shows the measurement result of the M toner as a representative example.
There can be understood from FIG. 6 that, as the reflectance of the intermediate transfer body becomes lower, a dynamic range of the density sensor output for the Bk toner becomes narrower. This means that density detection accuracy for the Bk toner is degraded.
That is, as the substrate density becomes higher since the residual toners not eliminated even by the cleaning are accumulated, the reflectance of the intermediate transfer body is degraded. Therefore, highly-accurate detection of a contrast of the Bk toner becomes impossible.
The same problem will occur not only for a printer of a type (i.e., batch transfer type) having such the intermediate transfer body as described above, but also for a printer of a type (i.e., multiple transfer type) in which the toner image on the photosensitive drum is overlapped each other on a recording sheet or paper sequentially for each color.
On the other hand, as to the patch formation onto the intermediate transfer body and the sequence in the patch density measurement, there has been desired an effective method which is inherent in the printer of the type using the intermediate transfer body.